At a production site, a remote master station and a plurality of sequencers (remote I/O stations) are connected using a transmission cable and the like to enable the remote master station to communicate with each remote I/O station, configuring a network to control various devices such as a solenoid valve, a motor, and a sensor connected to the remote I/O stations. That is, the remote master station controls and monitors the devices connected to each remote I/O station through the remote I/O station.
In addition, the remote I/O station includes an input detection circuit to detect a fact that input units such as a push button, a lever and a switch were operated by a user. This input detection unit corresponds to one input unit (for example, a push button), when the push button is pressed, a plurality of switches are turned on and input detection signals up to the number of switches are output. That is, the input detection circuit outputs a fact that one input unit was operated by making the plurality of the input detection signals redundant.
All such redundant input detection signals have a same polarity if the push button and the input detection circuit (such as a switch or a wiring) are normal; however, for example, if a foreign substance enters in one of the switches in the input detection circuit, or a wiring with a power supply or a ground is broken, at least one polarity of the redundant input detection signals might differ from other input detection signals. As a conventional technique to detect such a state, that is, an abnormality of redundant input detection signals, for example, Patent Documents 1 is known.
In order to recognize an error of two signal transmitters having a double redundancy of the automatic facility and find the position thereof, Patent Documents 1 discloses a technique related to an error detection recognition method of a signal transmitter having a redundancy for an automatic facility, which automatically recognizes not only a conducting wire failure but also the error of the transmitters at a high rate and finds the position thereof.
Specifically, a difference detector, which is an exclusive OR (XOR) element, detects whether or not signals from two signal transmitters accord with each other, and if the signals do not accord with each other, a timer device is started to start a measurement of a wait time T, and when the measurement of the wait time T is finished, a judgment whether or not the signals from the two signal transmitters are normal is performed.
Patent Document 1: JPA H5-225481 (KOKAI)